This invention concerns a digitized linearizing correction apparatus for an analog measuring instrument, and more particularly, a digitized linearizing apparatus utilizing frequency modulation to linearize the nonlinear input.
The nonlinear analog output response of thermocouples, and for that matter other types of transducers, long has been recognized. As technology progressed, the need for greater measurement accuracy proportionately increased, such that at the present time the error caused by the nonlinear output response of transducers has become significant. Often, because of the nature of an input transducer, errors are progressive over a known range and, with a linear indicator, a tolerable degree of error at the low end of the range soon changes to an unacceptable quantum of error by midrange and thereabove. Often the error is monotonic in nature, as is known in thermocouples.
Such nonlinearity has been dealt with previously by the use of various means including compensating meter movements which are designed to possess an equal, yet opposite nonlinear response characteristic. However, a meter is limited in its linear accuracy, especially when the linear approximation changes from one part or section of a response to the next. Nonlinear electronic elements, such as diodes and potentiometers and also servo-mechanisms of varying complexity and cost have been employed for linearization. Linearization often has been accomplished in an analog method; however, this results in apparatuses which are typically costly, occupy a relatively large amount of space, are slow to react and are subject to the inertia disadvantage of overshoot.
Most, if not all, of these deficiencies can be overcome by analog to digital conversion and digitized linearization of the error by providing discrete amounts of correction for application progressively, when needed. One system utilizing digitized linearizing correction is shown in U.S. Pat. No. 3,793,630 owned by the Assignee of the instant application. The system feeds a continuous train of clock pulses into counters and excludes certain of the clock pulses over the various ranges of the system such that the monotonic error progression is suppressed. This is an especially accurate method which can be used where mobility is not of a great deal of concern. However, the circuit required to produce this accuracy is too complex, too large and too expensive to be applied in a portable measuring system or instrument. This is especially true in producing a hand held instrument which must be packaged in a very small and lightweight package. To produce a linearizer for a hand held instrument, some accuracy can be sacrificed to provide an apparatus with fewer and less complex elements. This results in the required lightweight and smaller size apparatus and also a less expensive apparatus.